1. Field of the Invention
The present invention relates to a loop test system for testing the operation of an LSI (large scale integrated circuit) having a function of receiving data such as communication data and outputting it to an external circuit so that the external circuit can perform data processing, and a function of receiving the data from the external circuit and transmitting.
2. Description of the Related Art
In the data communication field, many LSIs have been developed which have the above function of receiving communication data or the like and outputting it to an external circuit so that the external circuit can perform data processing, and the function of receiving transmission data from the external circuit and transmitting it.
In some application fields, such as when an LSI is disposed on a line of a data communication network, an LSI self-test is frequently indispensable to ensure system performance.
This test must determine whether or not the LSI correctly receives the reception data and whether or not the LSI correctly transmits the transmission data.
Thus, an LSI having a data transmission and a reception function is provided with a RAM (random access memory) at its inside or outside. This RAM temporarily stores reception data and transmission data. If the formats of the reception data and the transmission data are the same, test data is set in the transmission data area of the RAM and then output from the LSI. The output data is then returned to the receiving side of the LSI and stored in the reception data area of the RAM. The contents of the reception data area and the contents of the transmission area are then compared. Thus, a self-diagnosis test is carried out by the LSI alone without requiring an interface for receiving test data. This might be called a loop test.
The phase of the reception data (hereinafter referred to as reception data RDAT) and the phase of the transmission data (hereinafter referred to as transmission data SDAT) treated by the LSI do not always exactly coincide. The reception data RDAT and the transmission data SDAT are time division data of which a plurality of lines are multiplexed. As shown in FIG. 2, reception data RDAT of FIG. 2C is transferred in bit units (B15, B14, . . . ) in synchronism with a predetermined reception clock of, for example, FIG. 2A. Further, the reception data RDAT is transferred in synchronism with the predetermined frame clock of FIG. 2B. Thus, information of a certain unit is transferred. As shown in FIG. 2D, the transmission data SDAT is transferred with its phase advanced by 2 bits from the phase of the reception data RDAT of FIG. 2C.
Accordingly, if the transmission data SDAT is simply returned to the receiving side of the LSI, the LSI cannot correctly receive it as reception data RDAT. Therefore, as shown in FIG. 1, an additional test circuit 102 for returning the transmission data SDAT to a reception side LSI 101 must be conventionally provided at the outside of the LSI 101.
More specifically, test data output from the LSI 101 as transmission data SDAT is supplied to a phase difference correcting circuit 103 composed of, for example, a plurality of flip-flops (F/Fs) within the additional test circuit 102. This correcting circuit 103 matches the phase of the transmission data SDAT with that of the reception data RDAT and then inputs it through a selector 104 to the receiving side of the LSI 101. In normal operation, the selector 104 is switched by a switching signal 105, and the reception data RDAT is input from the outside through the selector 104 to the receiving side of the LSI 101.
As described above, when there is a phase difference between the reception data RDAT and the transmission data SDAT, a circuit such as the additional test circuit 102 must be provided to absorb it. Therefore, when the test is performed with the system including the LSI 101 operating, a cumbersome procedure is needed.
Further, if the phase difference is large, the scale of the additional test circuit 102 must be very large.
Furthermore, in an LSI having the above data input and output functions, data formats of the reception data RDAT and the transmission data SDAT are not always the same. Therefore, reception and the transmission operations are often carried out in different formats due to differences among lines connected to the outside or the like.
In this case, even if the test additional circuit 102 shown in FIG. 1 returns the transmission data SDAT, the LSI 101 cannot receive it, so the test cannot be performed by the LSI alone. Consequently, as shown in FIG. 1, an emulator 106 exclusively used to test the transmission data SDAT or the like must be provided, making a complex test procedure unavoidable.
The increased scale of the test apparatus and the complicated test procedure are large obstacles to development of a self-test of an on-line LSI, as in the field of data communications.